Electrically actuated recording unit



April 1950 w. D. CANNON EI'AL ELECTRICALLY ACTUATED RECORDING UNIT 4Sheets-Sheet 1 Filed Feb. 3. 1945 IIHIIHIII INVENTORS W. D. CA N NON BYM. CANTOR ATTORNEY W. D. CANNON ETAL.

ELECTRICALLY ACTUATED RECORDING UNIT 4 Sheets-Sheet 2 April 4, 1950Filed Feb. 3. 1945 F'IG.2

INVENTORS W. D. CANNOI M. CANTOR BY mm ATTORNEY April 4, 1950 w. D.CANNON ET L ELECTRICALLY ACTUATED RECORDING UNIT 4 Sheets-Sheet 5 FiledFeb. 3. 1945 INVENTOR t;

W. DYCANNJN ANTO'i BY M C 1 Wi/Lwm ATTORNEY April 4, 1950 W. D. CANNONETAL ELECTRICALLY ACTUATED RECORDING UNIT Filed Feb. 3, 1945 4Sheets-Sheet 4 :-g to A 91 g 9 .z j g P" O :I 5 K lllllll v 6 HD 01 Jr)A :2: '2.

IL INVENTORS w. 0. CANNON M. CANTOR BY ATTORNEY Patented A r. 4, i980.UNlTED srArss PATENT "orncs uoauo I William D. Cannon, Metuehen, N. 1.,and Michael Cantor, Brooklyn, N. Y., million to The Western- UnionTelegraph Company, New York,- N. Y., a corporation oi New York mum.February s, 1045, serial No. stem 11- claim. 1

This invention relates generally to an elecfrequencies extending-fromapproximately zerjc to a hundred and fifty cyclesn Whilethe device issuitable for the recording of small variable currents fromany source, ithas particular application in the medical field, for example, inelectrocardiographs, encephalographs, and other similar devices forrecording electrical reactions resulting from biologic processes.

' Heretofore the most-common type of such instrumentsfor example, theelectrocardi'ograph, usually has comprised a sensitive string galvanometer arranged for projecting a beam of light onto moving photographicfilm. The galvanometer, however, is delicate, easily damaged, and isdifficult to repair, and'since the operating currents are very minute,the consequent hypersensitivity of the movement has in general precludedthe use of recording methods other than photographic. Photographicrecording also is objectionable becauseof limitations imposed upon theconstruction of the recorder, the imposition of the necessary developingprocess, and the time consumed therein. 4

The present invention involves a recording unit which, while extremelysensitive, is rugged and portable, does not depend upon the photographicprocess, and moreover is direct reading so that the record obtained maybe observed during the recording operation. It is also desirable toemment that is critically damped in a manner to avoid frequencydistortion throughout the entire range of frequencies for which the unitis to be used Another object is a recorder unit in which the control ofthe moving coil motional impedance is eflected by controlling the fluxdensity or the field oi the electromagnetic'driving unit for therecording stylus.

A further object is to provide a lateral stylus movement which is linearwith respect to the 1 magnitude of the actuating current throughout theamplitude of travel of the stylus, and-in which long uniform travel ofthe stylus is chtained. I

light construction with a constant current-source Additional objectscomprise the matching of the moving coil motional impedance withtheoutput impedance 'of the associated driving amplifier; to providearugged flexible stylus of for energizing the stylus; to align thecentral plane of the moving coil of the electromagnetic unit withrespect to the pivotal or flexing point of the recording stylus arm; andto provide suit- 5 able means for quickly and easily calibrating theapparatus prior to a recording operation.

The foregoing and other objects and advantages of the invention will beapparent from the following detailed description, taken in connectionwith the accompanying drawings, in

which:

Fig. 1 is a plan view of an electrocardiograph tape recorder unitembodying the principles of ploy an electrosensitive' recording blankwhich requires no special treatment either before or after the recordingoperation, and which is un- 'aflected by varying conditions of light,heat,

humidity, and the like. One form of a dry electrosensitive recordingpaper which is suitable for the purposes of the instant invention is apaper such as disclosed in Patents Nos. 2,294,146,

' 2,294,147, and 2,251,742, although other known types ofrecordingpapers suitable for the purpose p having 8 graph co ded e n y t may beemployed. Preferably the conducting paper disclosed in the aforesaidPatent No. 2,251,742 is used since the paper islight in color, isrelatively inexpensive, and is adapted for the invention;

Fig. 2 is a front view, in elevation, of the unit of Fig. 1;

Fig. 3 is a view in elevation of the left hand end of the unit as viewedin Fig. 2;

Fig. 4 is a sectional view, taken along the line 40 4-4 of Fig. 1;

Fig. 5 is a circuit diagram of the electrical sys tem of the recordingunit and associated amplitier; and

' Fig. 6 shows a length of the electrosensitive recording unit of Figs.1 to 5.

Referring particularly to Figs, 1 and 2 of the.

drawings, the unit is shownas comprising a hollow mounting base It onwhich is secured a plate making permanent fine line records at very hi hii on which the various elements of the unit are speeds from a properlydesigned and energized electrical stylus.

One of the objects of the invention is an electrical recording unit ofthe character described mounted. The bottom of the housing It is closedby a plate l2, and to this plate are secured pads- H of felt or othersuitable material to prevent scratching of the desk or table on whichthe which has an electromagnetic moving coil ele-i unit may be pla d- Onthe plate I l is mounted a bracket member l1 having a projecting coremember I!) on which is rotatably mounted a roll R of electrosensitiverecording tape t, such as the lead thiosulphate paper disclosed in theaforesaid Patent No. 2,251,742. The roll of tape is slipped over theforward end of the core l9, and is prevented from coming off by aknurled screw Hi, there being sufficient clearance between the sides ofthe roll R and the bracket member I! and knurled knob ill to enable theroll of tape lib revolve freely about the core. From the roll R the tapet passes around a tape guide member 20 supported by an upstandingbracket 2|. From the tape guide member 20, the tape passes around a tapeguide roller 22 mounted on a member 23 so that the roller is askew withrespect to the guide member 20, thereby to deflect the tape, as seenmore clearly in Fig. 3. so that the center of the tape will be in properalignment with the recording apparatus, as hereinafter described.

From the guide roller 22 the tape is passed around a roller 24 which isrotatably mounted on a plate or bracket member 25, Figs. 1 and 2, andthence over a platen 26, the platen having sides 21 secured thereto tomaintain the tape in proper position during the recording operation.From the platen 26 the tape passes around the guide roller 28 and thencebetween knurled feed rollers 29 and 32, the knurled portions of theserollers extending only a comparatively small distance across the edgesof the tape so that any indentations formed in the tape by the rollerswill not aile'ct the readibility of the graph recorded thereon. The feedroller 29 is mounted on a shaft 30 which, at its outer end, is supportedby bracket member 3|. The pressure roller 32 is carried by a bracketmember 33 which is secured to a bell crank 34 that is pivotally mountedon the plate or bracket 25. A helical spring 35 provides the properresilient tension for the pressure roller 32. The feed roller 29 isdriven, through a flexible coupling 36, Fig. 1, from the shaft 31 of aconstant speed tape pulling motor 38, which motor may be either thealternating current or direct current type, and energized from a localsource of commercial power.

Bearing on the tape t is a flexible stylus arm or nib 4B of the lightestpossible construction. Preferably, it is composed of a strip of lightflexible metal of nickel iron, nickel steel or the like, and has its endturned down and sharpened to provide a stylus point of small diameterwhere it contacts the recording tape t; a diameter of the order of eightthousandthsof an inch is satisfactory for fine recording. While thestylus wear has been found to be quite small, nevertheless it is desiredto have it replaceable, and the stylus is frictionally inserted within alight weight stylus arm 42, preferably composed of aluminum, the armhaving a U-shaped cross section for the pivoted end of the arm, in orderto provide the desired rigidity thereto without adding appreciableweight to the arm, the rib portion 43, seen more clearly in Fig. 4,serving to stiffen the arm 42. The stylus arm is secured to a mountingblock 44, and isinslllated therefrom as shown in Figs. 1 and 3 of thedrawings, the mounting block 44 being secured to a bracket 45 which, inturn, is carried by a cradle member 48 that supports the electromagneticdriving unit 48.

The electromagnetic driving unit 48 comprises a cylindrical core 59composed of material of high magnetic permeability, the core having aforward end 60 thereof which forms a large cyllndrlcal pole piece forthe unit. Surrounding the core 59 is a field winding 6| having a largenumber of turns. Surrounding the cylindrical pole piece 60 is acylindrical moving coil 65, having a large number of turns of fine wire,secured to and carried by a circular light weight fiber member 66, theouter end of which is secured to the stylus arm 42 for driving thestylus 40. An intense magnetic field is produced across the air gap bythe field winding 6|, the magnetic circuit including the outer shell ofthe unit and an annular ring 62. The operating air gap must be ofsufficient width to accommodate the movable coil 65, and must be ofsubstantial length so that the coil 65 at the extremes of its movementwill still encounter a magnetic field of uniform strength. To effectthis the annular pole piece 65 is formed or cut away at B3 to give areduced length 64' of air gap on the pivot side of the stylus member 42,so that when the annular coil is actuated to either extreme positionsuch that it is flush with the edge 64, it will also be flush with theedge 64'. the arrangement making more efficient use of the totalavailable flux. This means that the total amount of flux available atthe longer edge 64 is somewhat greater than that at the opposite shorteredge 64, and thus makes it possible to increase the damping effect andenable the use of a stronger spring tension with the stylus driving arm42. It will be seen from Fig. 4 that the rib 43 of the stylus terminatesat a point at, and .hat the insulating plate 44a is grooved at thispoint, so that the stylus arm pivots or flexes at the point .23. As seenin Fig. 1, this point lies in an imaginary plane that is perpendicularto the longitudinal axis of the coil 65 and which intersects the coil 65midway between the ends thereof, so that maximum recording movement ofthe coil may occur before it strikes either of the pole piece edges 64or 64'. Moreover, the fixed portion of the stylus member 42 is somounted that the point x, Figs. 1 and 4, about which the stylus flexes,and the median position of the free end of the stylus lie in animaginary plane that is normal to the surface of the recording strip andparallel to an edge thereof at the recording position, so that the endof the stylus 40 in its travel will not introduce appreciable distortionin the recorded graph. Preferably, this plane coincides with the centerof the tape t. A stylus stop arm 61, Figs. 1, 3 and 4, limits themaximum extent of movement of the stylus in either direction of itstravel.

Since the weight of the moving coil comprises a limitation upon theupper frequency to which it will respond, light weight is a paramountconsideration. At the same time the coil must be energized by amplifierpower tubes whose output impedance is normally of substantial value andpreferably without the interposition of transformers. A proper matchingof the coil and amplifier impedance necessitates a large number of turnson the coil. In the specific embodiment illustrated this has beenaccomplished through the winding of approximately 2500 turns of No. 42enamelled copper wire into a cross section of approximately by Aluminumor other wire may also be used for this purpose. The wire is wound uponthe circular form 66 of thin fiber or other suitable mechanically stronginsulating material.

An amplifier, preferably enclosed within the housing 10, is associatedwith the recorder for the p rp se of amplifying the minute variable wasgenerated by the biologic processes arcane cient to drive the recordingunit described. Obviously the amplifier should leave no impression ofits own characteristics upon these currents.

The amplifier should not pass direct current but I including a uniquecalibration control.

Referring to Fig. there are disclosed at the upper left-hand portion ofthe figure a plurality of electrodes IIO for application to variousportions of the body of the subject such as the left arm, left leg,right arm and chest in the case of a cardiograph, those electrodes beingdesignated by the letters .LA, LL, RA and CH to indicate the .variousportions of the body of the subject to which they may be applied. Theelectrodes are connected to banks of contacts III and H2, and morespecifically to arcuate contacts 2 to 5 of the banks. Contacts I of thebanks are connected by means of conductors Ill and H5 to a calibratingcircuit hereinafter described. The amplifier disclosed is a three-stage,push-pull, resistance-coupled amplifier, the input of which isconnected, by a double-contact switch compri i g switch blades Iii, tothe source of potentials to be measured, that is, to pairs of contacts 2to I of the banks III and'I I2. As hereinafter appears, the symmetricalstructure of the amplifiers imparts substantial immunity to interferingfields, stray currents and power supply noise which would tend to afiectboth sides of the amplifier with respect to ground. This cancellation ofinterference is rendered more effective through the use of thesymmetrical, relatively low value resistances I20 across its inputcircuit, and by the use of the large common resistances I2I and I22respectively in the connections to the cathodes and screen grids of thefirst stage. These resistances cause degenerative amplification of thelongitudinal currents but do not affect the desired series currents.

The first and second stages of the amplifier comprise pairs of tubes IISand III, and the third or power stage of the amplifier comprises a pairof tubes Hi, all of these tubes being connected in push-pull relation;Tubes IIIi preferably are the type known as No. 6J7; tubes III are ofthe type known as No. 68.1.7; and tubes .III are of the type known asNo. 6Y6; other types of tubes suitable for the purpose may, of course,be'employed. For the sake of simplicity, the conventional filaments orheater elements associated with, the indirect heated cathodes of thetubes are omitted in the drawing.

In amplifiers of the type illustrated the interstage coupling condenserstend to discriminate against lower frequencies, which efi'ect iscompensated for by the use of simple resistance-capacity networksconnected in shunt which tend to discriminate against the higherfrequencies. In such sensitive amplifiers, when potentials ofappreciable value are inadvertently impressed upon theinput circuit,substantial voltages are impressed upon the grids of the amplifier tubeswhere they are isolated by the coupling conis made.

This action Is substantially eliminated by inserted large sizeresistances I20 and m' in series with the grids of the tubes, inconjunction with the proper choice of grid leak resistances I20 and I2l.The value of each of the resistances I28 and I2! may, for example, be ofthe order of 200,000 ohms, while the values of the grid leak resistancesI2! and IN will be of the order of one megohm each. These values of theresistances I28 and I2! keep the tube impedance high even when the gridis positive with respect to the cathode, and prevents the condensers I21and I21 from charging up at this time. In consequence, accidental largevoltages which would initially drive the stylus beyond the limits of therecording tape are quickly dissipated so that the stylus immediatelyreturns to the center of the tape to resume accurate and normalrecording of the impressed bioelectric potentials.

The size of the coupling condensers I21 and I21 relative to theresistances I25 and I2! is such that the condensers will pass the lowestfrequency involved in cardiograph potentials with out appreciabledistortion. To further improve the amplifier in regard to low frequencyresponse, condensers I30 and Ill and resistances iii and Ill areinserted across the two grid circuits of the second and third stages.

The plate resistances I32 and I32 in the first and second stages may beof the order of 300,000 ohms; the screen grid resistance I22 of thefirst stage may be of the order of 400,000 ohms, and the correspondinggrid resistance of the second stage may be of the order of one-halfmegohm. .The plate resistances I33 are of lower value because they arein the power stage of the amplifier; a suitable value of theseresistances is of the order of 1500 ohms each.

The second stage gain control is obtained by controlling thedegenerative feedback without changing the bias on the tubes, and thisprevents violent swing of the stylus when a change in gain ResistancesIIII with a slider III are provided for this purpose, which resistancesare connected to ground through resistance 2. Shifting the sliderchanges the amount of gain. with the maximum gain being obtained whenthe slider is moved to the left as viewed in the figure. The values ofthe resistances are proportioned to give the desired amount of control;in the embodiment disclosed each of the resistances I40 and I42 may havea value of the order of 500 ohms. Resistances I2I and II! are providedin the first and third stages, respectively, to provide suitable cathodebias. The output of the amplifier from the terminal resistances its isconnected directly to the movable coil of the electromagnetic unit.

Power for energizing the amplifier is supplied from a source ofalternating current in transformer winding I50. Associated with thiswindplied to the plate and screen grid circuits oithe amplifier tubes,through a smoothing circuit comprising an inductance I50 and groundedcon denser' Ill. A current limiting resistance III is densers until asuificient time has elapsed for inserted in conductor III which suppliescurrent to the plate and screen grid circuits of the first amplifierstage and the screen grid circuits of the second stage. Connected toconductor III is a gas-filled voltage regulator tube 50 which stabilizesthe plate and screen grid voltages. If, for example. the voltage in thewer supply lead I59 should increase, the increased current passesthrough the tube 50 without causing any sub- 1 stantial increase orchange in voltage drop across the tube.

It is highly desirable to insure uniformity of marking on the sensitizedrecording paper tape or strip as the stylus is actuated at varying ratesof speed depending upon the value of the recorded signals. Duringoscillation or lateral movement of the stylus, it is apparent that themarking time is much less than when straightaway or relativelongitudinal movement alone prevails between the stylus and tape. Fig.6-

illustrates a cardiagram in which uniformity of marking under theseconditions of varying contact time is accomplished through the use-of aconstant current feed to the stylus, and Fig. 5 illustrates how thiseffect is secured. The stylus energizing current is obtained fromtransformer winding I5I, through a full wave rectifier I55. The directcurrent output circuit of the rectifier has a grounded filter condenserI60 connected thereto, and the circuit to the stylus 40 has connected inseries therewith a resistance I6I and an inductance I52. resistance maybe of the order of one-tenth of a megohm, and the inductance may have avalue of the order of thirty henries or more. A potential ofapproximately three hundred volts is employed, and under theseconditions a current of approximately three milliamperes flowscontinuously in the stylus circuit, irrespective of the movement of thestylus or the varying resistance of the recording paper, these valuesobtaining when a recording paper of the type disclosed and claimed inthe aforesaid Patent 2,251,742 is employed. If for any resason the tapet moves very slowly or stops, the impressed voltage is not suflficientto burn or otherwise impair or disfigure the graph or other record, andthe record will be represented by a line of uniform density and width.

Current for energizing the field winding 6| of the electromagnetic unitis obtained from the rectifier I52, the circuit including a variableresistance I65, for controlling within close limits the current flowingin the field coil and thus maintain a flux density of predeterminedvalue in the operating air gap to control the moving coil motionalimpedance and critically dampen the moving coil and stylus to avoidfrequency distortion throughout the range of low frequencies applied tothe recorder. Preferably the field energizing circuit has a groundedfilter condenser I58 connected thereto. Due to the fact that the movingcoil motional impedance substantially matches the output impedance ofthe amplifier, the output circuit of the amplifier may be connecteddirectly to the moving coil 65, as illustrated in Fig. 5, without theinterposition of a transformer.

Prior to each recording operation it is desirable to calibrate theamplifier and recording circuit. In accordance with the presentinvention there is provided a 'simple calibrating arrangement whichrequires no batteries to furnish a standard potential, and which ispermanently accurate. The voltage across the gas tube 50, connectedbetween the plate supply and ground, is utilized as a source of standardpotential. A portion of this voltage is obtained by means of apotentiometer I", I'll, and is used to charge In the circuitillustrated, the

a condenser I12 which provides a source of unit input calibratingpotential whenever conductors Ill and III are connected to the input ofthe amplifier by means of the switch blades or connectors Ill, and thecalibrating push button? is depressed, thereby connecting the chargedcon-- 1 denser across the input of the amplifier. The gain control I40,I in the intermediate stage of the amplifier is then adiusted until thedesired amplitude of the lateral movement of the stylus corresponding tothe unit input potential is obtained. This relationship may, forexample, be of the order of one centimeter lateral deflection for onemillivolt of applied voltage. when the peak valueof the wave form in notequal to a standard or predetermined desired height, the gain control isadjusted until the desired peak values are obtained. The precalibratingvoltage applied by the resistances or potentiometer I'll, III to thecondenser I12 cause the condenser to produce a wave form ofexponentialdecaying, thus producing a peak of extremely short durationfor calibrating purposes. An on-oif recording switch S is provided for'the recorder; when the switch is in its closed position shown in Fig. 5,it will be seen that it short-circuits the grid circuits of the lastamplifier stage, and that when the switch is moved to its open position,the unit is in condition for recording.

Reference has heretofore been made to the necessity for matching theimpedance of the output stage of the amplifier to that of the movingcoil of the recording unit, this being of particular importance inconnection with the maintenance of proper frequency response. In theabsence of damping measures, the moving element of the recording unitwill tend to respond more freely at its natural period which normallylies at a frequency near the upper limit of its useful range, whicheilect in itself is productive of some distortion. At the same time themoving coil produces a back E. M. F. as a consequence of its motion inthe magnetic field. This voltage is applied across the amplifier outputcircuit which consequently acts as a damping impedance. If the amplifieroutput tubes are of high impedance, the damping is consequently low andthe response of the unit to transients will be impaired. On the otherhand, if the tube impedance is low, the coil movement will beover-damped and sluggish.

Considerable advantage accrues if an approximate match of the impedanceof the amplifier output stage with the motional impedance of the movingcoil is brought about through control of the design of the two entities.The motional impedance of the coil corresponds to the back E. M. F.generated by the coil and is controlled in part by the strength of thepermanent field in the air gap. It is possible, therefore, to adjust themotional impedance by current control means in the exciting fieldcircuit. and this adjustment is made initially and maintained by meansof the resistance I65 hereinbefore referred to. so that eillcient andstable operation of the combined amplifier and recording unit isachieved.

The recording device has been described with particular reference tomedical applications. The device, however, is of general applicabilityto any purpose which requires the recording of sensitive currentmanifestations, or where the currents are of sufficient magnitude theamplifier may be omitted and the recorder unit used alone. Illustrativeapplications are as a recorder of submarine cable signals, as anoscillograph, or for the recording of atmospheric or other disturbances.

. 9 While the invention has been described in connection'with apreferred embodiment thereof. it will be understood that variousmodifications and 10 f movable coll for actuating the stylus. the numberof turns of said coilbeing such as to cause the on an electrosensitiverecording strip, comprising an electrical stylus of low inertia having afixed portion, a flexible portion and a free end portion for applyingmarking current to the re-' cording strip, means for applying a sourceof constant current to the stylus, an electromagnetic unit for drivingthe stylus, said unit comprising a magnetic core having a circular polepiece, an opposed pole piece surrounding said core and spaced therefromto form an operating air gap, an energizing field winding surroundingthe core and an annular movable coil disposed in the air gap between thepole pieces, support means secured to said stylus intermediate theflexible and free end portions thereof for carrying said movable coil,means comprising a source of constant current for exciting said fieldwinding, and means for applying said variable electrical currents tosaid movable coil for actuating the stylus.

2. Apparatus for recording variable electrical currents of smallmagnitude and low frequency on an electrosensitive recording strip,comprising an electrical stylus of low inertia having a fixed portion, aflexible portion and a free end portion for applying marking current tothe recording strip, means for applying a source of constant current tothe stylus, an electromagnetic unit for driving the stylus, said unitcomprising a magnetic core having a pole piece, an opposed pole piecesurrounding said core andspaced therefrom to form an operating air gap,an energizing field winding for the core and a movable coil disposed inthe air gap between the pole pieces, support means secured to saidstylus for carrying said movable coil, means comprising a source ofconstant current for exciting said field winding, means for applyingsaid variable electrical currents to said movable coil for actuating thestylus, the number of ampere turns of said field winding and the numberof turns of said movable coil being such as to generate aback. E. M.F.,of a value in said movable coil as to critically dampen the movingcoil and stylus to avoid frequency distortion throughout the range oflow frequencies for which the recorder is adapted.

3. Apparatus for recording variable electrical .currents of smallmagnitude and low frequency on an electrosensitive recording strip,comprising an electrical stylus of low inertia having a fixed portion, aflexible portion and a free end portion for applying marking current tothe recording strip, means for applying a source of constant current tothe stylus, an electromagnetic unit for driving the stylus, said unitcomprising a magnetic core having a pole piece, an opposed pole piecesurrounding said core and spaced therefrom to form an operating air gap,an energizing field winding for the 'core and a movable coil disposedinthe air'gap between the pole pieces, support means secured to saidstylus for carrying said movable coil, means comprising a source ofcon-, stant current for exciting said field winding, means including anamplifier whose output c rcuit is connected directly to said movablecell for applying said variable electrical currents t9 the moving coilmotional'impedance substantially to match the output impedance of saidamplifier to avoid frequency distortion throughout the range of lowfrequencies for which therrecorder is adapted.

4. Apparatus for recording variable electrical currents on anelectrosensitlve recording strip,

comprising an electrical stylus of low inertia having a fixed portion, aflexible portion and. a free end portion for applying marking current tothe recording strip, means for applying a source of marking current tothe stylus, an electromagnetic unit for driving the stylus, said unitcomprising a magnetic core having a pole piece, an opposed pole pieceadjacent to said core and spaced there-- l from to form an operating airgap. an energizing field winding for the core and a cylindricalmovable.coil disposed in the air gap between the pole pieces, supportmeans secured to said stylusin-v drical coil and which intersects saidcoil midway between the ends thereof. l

5. Apparatus for recording variable electrical currents on anelectrosens tive recording strip, comprising an electrical sty us of lowinertia having a fixed portion, a flexible o tion and a free end portionfor applying marking current to the recording strip, means for advancingsaid strip longitudinally along the stylus during a're'cordingoperation, means for applying a source -of marking current to thestylus, an electromagnetic unit for driving the stylus. said unit compriing a magnetic core having a pole piece, an op osed pole piece adjacentto said core and s aced there= from to form an o erating air gap. anenergiz ng fie d winding for the core and a movab e coil dis posed inthe air gap between the pole pieces. support means secured to saidstylus intermediate the flexible and free end thereof for carrying saidmovable coil, means comprising a source of current for exciting saidfield winding, means for applying said variable electrical currents tosaid movable coil for ac uating the stylus, and means for supporting thefixed portion of the stylus in such position that the fiexure point ofthe stylus and the median position of the free end thereof substantiallylie in an imaginary plane that is normal to the surface of the recordingstrip and I parallel to an edge thereof at the recording position. I I

6. Apparatus for recording variable electrical currents on anelectrosensitive recordin strip, comprising an electrical stylus 01' lowinertiahaving a fixed portion, a flexible portion and a free end portionfor applying marking current to the recording strip, means for advancingsaid strip longitudinally along the stylus during a recording operation,means for applying a source of marking current to the stylus, anelectromagnetic unit for driving the stylus, said unit comprising amagnetic core having a pole piece, an opposed pole I piece adjacenttothe first pole piece and spaced drical "movable coil disposed in the airgap between 11 the pole pieces, support means secured to said stylusintermediate the flexible and free end portions thereof for carryingsaid movable coil, means comprising a source of current for excitingsaid field winding, means for applying said variable electrical currentsto said movable coil for actuating the stylus, and means for supportingthe fixed portion of the stylus in such position that the fiexure pointof the stylus substantially lies in an imaginary plane that isperpendicular to the longitudinal axis of said cylindrical coil andwhich intersects said coil midway between the ends thereof, and saidflexure point and the median position of the free end of the stylussubstantially lie in an imaginary plane that is normal to the surface ofthe recording strip and parallel to an edge thereof at the recordingposition.

7. Apparatus for recording variable electrical currents on anelectrosensitive recording strip, comprising an electrical stylus of lowinertia having a pivotal portion and a free end portion for applyingmarking current to the recording strip, means for applying a source ofmarking current to the stylus. an electromagnetic unit for driving thestylus, said unit comprising a magnetic core having a circular polepiece, an opposed annular pole piece having an inner edge thereofsurrounding said core and spaced therefrom to form an operating air gap,an energizing field winding for the core and an annular movable coildisposed in the air gap between the pole pieces, support means securedto said stylus for carrying said movable coil, means comprising a sourceof cur-v rent for exciting said field winding, means'for applying saidvariable electrical currents to said movable coil for actuating thestylus, said annular pole piece having its said inner edge formed toprovide a shorter length on the side closest to the pivotal portion ofthestylus and a longer length on the side farthest from said pivotalportion of the stylus, thereby to increase the total flux efi'ective foractuating said movable coil throughout the extent of its movement.

8. Apparatus for recording variable electrical currents on anelectrosensitive recording strip, comprising an electrical stylus of lowinertia having a pivotal portion and a free end portion for applyingmarking current to the recording strip, means for applying a source ofmarking current to the stylus, an electromagnetic unit for driving thestylus, said unit comprising a magnetic core having a circular polepiece, an opposed annular pole piece having an inner circular edgesurrounding said core and spaced therefrom to form an operating air gap,an energizing field winding for the core and an annular movable coildisposed in the air gap between the pole pieces, support means securedto said stylus for carrying said movable coil, means comprising a sourceof current for exciting said field winding; means for applying saidvariable electrical currents to said movable cell for actuating thestylus, said annular pole piece having its said inner circular edgeformed to provide a shorter length on the side closest to the pivotalportion of the stylus, said edge progressively increasing in length fromsaid closest side to the side farthest from said pivotal portion of thestylus, thereby to increase the total flux effective for actuating saidmovable coil throughout the extent of its movement.

9. Apparatus for recording variable electrical currents of smallmagnitude and low frequency on an electrosensitive recording strip,comprising an electrical stylus of low inertia having a pivotal portionand a free end portion for apinductance in the circuit to maintain aconstant value of stylus current, an electromagnetic unit for drivingthe stylus, said unit comprising a magnetic core having -a circular polepiece, an opposed pole piece surrounding said core and spaced therefromto form an operating air gap, an energizing field winding surroundingthe core and an annular movable coil disposed in the air gap between thepole pieces, support means secured to said stylus intermediate theflexible and free end portions thereof for carrying said movable coil, acircuit for supplying constant current for exciting said field winding,an impedance in said field circuit for regulating the current flowingtherein, and means for applying said variable electrical currents tosaid movable coil for actuating the stylus.

10. Apparatus for recording variable electrical currents on anelectrosensitive recording strip, comprising an electrical stylus of lowinertia having a pivotal portion and a free end portion for applyingmarking current to the recording strip, an electromagnetic unit fordriving the stylus, said unit comprising opposed pole pieces spaced toform an operating air gap, and a movable coil disposed in the air gapbetween the pole pieces, support means secured to said stylus forcarrying said movable coil, means including a thermionic amplifier whoseoutput circuit is connected to said movable coil for applying saidvariable electrical currents to the movable coil for actuating thestylus, gain control means in the amplifier circuit. a power supplycircuit for said amplifier connected to a source of rectifiedalternating current, calibrating means comprising a gas-filled voltageregulator tube connected to said power supply circuit to provide asource of standard potential, means including a manually operable switchfor driving a predetermined calibating potential from said source ofstandard potential and applying the calibrating potential to the inputcircuit of the amplifier, and means for manually adjusting the gaincontrol until a desired deflection of said recording styluscorresponding to the unit input potential is obtained.

11. Apparatus for recording variable electrical currents on anelectrosensitive recording strip, comprising an electrical stylus of lowinertia having a pivotal portion and a free end portion for applyingmarking current to the recording strip, an electromagnetic unit fordriving the stylus, said unit comprising opposed pole pieces spaced toform an operating air gap, and a movable coil disposed in the air gapbetween the pole pieces. support means secured to said stylusforcarrying I said movable coil, means including a thermionic amplifierwhose output circuit is connected to said movable coil for applying saidvariable electrical currents to the movable coil for actuating thestylus, gain control means in the amplifier circuit, a power supplycircuit for said amplifier connected to a source of rectifiedalternating current, calibrating means comprising a gas-filled voltageregulator tube connected to said power supply circuit to provide asource of standard potential, a condenser and means including contactsof a manually operable switch for charging said condenser from saidsource of standard potential, means including other contacts of saidswitch for connecting momentarily the charged 13 4 r 1 condenser to theinput circuit of the amplifier, and UNITED STATES PATENTS means formanually adjusting the gain control 7 Number Name D t until a desireddeflection of said recording stylus 473,538 Weston 26, "92 correspondingto the uni inpu potential is 613,003 Andrews t, 25,1893 tained- 51,986,583 King Jan. 1, 1935 WIILIAM CANNON 2,049,724 Thomas 4 Aug. 4,1938 MICHAEL CANTOR- 2,170,515 Dionne Aug. 22, 1939 2,267,465 JakoskyDec. 23, 1941 REFERENCES CITED 2,301,220 Lowe Nov. 10,1942 The followingreferences are of record in the 2,306,456 Mayne Dec.29,1942

file of this patent: 2,351,353 McCarty June '13, 1944

